The invention relates to a device for transmitting electric current between two components, which can rotate with respect to one another, of a steering device for motor vehicles.
Apparatuses such as these are used for transmitting electric current (in particular signal current and/or power current) from the vehicle electronics (which are arranged fixed to the bodywork) of a motor vehicle to the steering wheel, and vice versa. This allows electrical or electronic functional elements which are provided on the steering wheel, such as the trigger for the gas generator of an airbag unit, steering wheel heating or electrical switches for operating indicator lights, windshield wipers or a radio, to be integrated in the steering wheel and to be supplied with current from the vehicle electronics.
A device such as this for transmitting electric current is known from DE 41 11 699. This device has a rotor which is associated with the steering wheel and can rotate together with it, as well as a stator which is associated with an assembly whose position is fixed with respect to the rotary movement of the steering wheel, for example the steering column cladding of the steering device. The rotor and the stator are connected to one another by means of a flexible electrical conductor, which is in the form of a strip-like printed circuit. This flexible electrical conductor is rolled up on or unrolled from an intermediate panel, which is formed between the rotor and the stator, depending on the rotation direction of the rotor.
In the known apparatus, the number of individual conductor tracks in the printed circuit on the flexible conductor is restricted by its dimensions. If more conductor tracks are required, then either a larger flexible conductor or a number of conductors must be used. In both cases, this leads to an increased amount of space being required for the flexible conductor in the apparatus.
One object of the invention is to provide a device for transmitting electric current which provides a great a number of conductor tracks for current transmission as possible, while requiring little space.
According to an embodiment of the invention, an apparatus for transmitting electric current between two components (a stator and a rotor), which can rotate with respect to one another is provided.
According to this embodiment, the flexible conductor which runs from the stator to the rotor is in the form of a flexible printed circuit. When the steering wheel is rotated, this flexible printed circuit can be wound up onto or unwound from a winding element, and has a flat flexible substrate, on whose front face and rear face conductor tracks run from the stator to the rotor.
As a flexible substrate, a flexible printed circuit such as this has, for example, an electrically insulating substrate film which is provided with a number of metallic conductor tracks on both sides, with the conductor tracks being covered by insulator films.
In order to produce the desired conductor track design, areas of the thin metal layers which are not required in this case on the front face and rear face of the flexible substrate film are removed, for example by a photolithographic and etching technique, so that the desired individual conductor tracks are produced.
The use on both sides of at least one substrate film as a flexible printed circuit thus makes it possible, in a simple manner, to double the surface area to which the desired circuit and conductor track design can be applied.
Furthermore, in addition to transmitting current, the flexible printed circuit can also carry out other functions, by integrating further electrical or electronic components in the printed circuit. This reduces not only the number of separate electrical and electronic components in the area of the steering device, but also the space that they require. Thus, in addition to transmitting electrical signals from the stator to the rotor, a single printed circuit can also carry out a large number of additional functions.
The flexible printed circuit has in each case at least one associated electrical connection on the stator and on the rotor, with the flexible printed circuit having electrical contact elements, which can make electrical contact with these connections, at its stator end and at its rotor end. Said contact elements are preferably in the form of plug elements, which can be inserted into the corresponding electrical connections on the stator and on the rotor, and are connected to the conductor tracks on the flexible substrate by means of electrical lines.
The contact elements for the flexible printed circuit may in this case be provided both for making contact with electrical connections which are arranged essentially transversely with respect to the direction in which the flexible printed circuit extends and for making contact with electrical connections which are arranged essentially parallel to the flexible printed circuit.
If a large number of conductor tracks are required between the stator and the rotor, then it is advantageous to design the conductor tracks (which, for example, are produced by printing a metal layer on both sides of the flexible substrate film, followed by selective etching) such that the distance between the individual conductor tracks is greater in the region of the rotor end and/or of the stator end of the printed circuit than in the central section of the printed circuit. This allows the central section of the flexible printed circuit, which is wound up and unwound during rotation of the steering wheel, to be made particularly narrow. The flexible printed circuit then broadens out at both of its ends, so that the individual conductor tracks there can be arranged further apart from one another, thus making it easier to make electrical contact with the individual conductor tracks.
It is also advantageous for the flexible printed circuit to have reinforcing elements at its stator end and/or at its rotor end, in particular in the region of its electrical contact elements, in order to give it robustness. These reinforcing elements may be formed, in particular, by plastic elements, which are molded or clipped onto the flexible printed circuit. FR4 boards are particularly suitable for use as reinforcing elements, which specifically support the flexible substrate of the printed circuit, which is preferably composed of polyester or polyimide.
The flexible printed circuit preferably has means to ensure strain relief for the electrical contact between the electrical lines and the conductor tracks on the flexible substrate film of the printed circuit. By way of example, the reinforced regions may have a holding section which has a number of guide cutouts, into which the electrical lines fit and can be connected with a force fit to the reinforcing elements.
In order to simplify the installation of said electrical contacts, it is advantageous for the reinforcing elements to comprise at least two individual elements, which are in the form of small panels, are essentially of the same size, and can be joined together such that the flexible printed circuit and the electrical lines that are supplied to it are enclosed.
The individual elements can be attached to one another at the joint edges by means of film hinges, so that the flexible printed circuit and the electrical contact elements can be enclosed by the individual elements in a simple manner, by means of a folding mechanism.
The flexible printed circuit can either be connected to the stator and to the rotor, respectively, at its ends via latching elements, or else can be adhesively bonded to the stator and to the rotor, respectively. The connection is in this case advantageously provided via said reinforcing elements.
A further embodiment of the invention has a broad rotor end and/or stator end on the flexible printed circuit, with the conductor tracks which run on the front face and rear face of the flexible printed circuit ending in regions which are arranged alongside one another on the flexible printed circuit. Electrical contact elements can thus be attached to each of these conductor track ends without there being any risk of a short circuit between the conductor tracks on the front face and rear face of the flexible conductor during the contact-making process. The rotor end and/or stator end of the flexible printed circuit can be folded such that the electrical contact elements for the conductor tracks come to rest alongside one another transversely with respect to the direction in which the printed circuit extends, and thus form a compact electrical contact element.
The flexible printed circuit can also be provided with increased strength and robustness by providing it with a plastic sheath, at least in places.
A further major advantage of the solution according to the invention is that further electronic assemblies can be integrated in the flexible printed circuit, in particular at one of its ends.
For example, a steering angle sensor, whose electronic evaluation unit is integrated in the flexible circuit, can be provided at the stator end of the flexible printed circuit. An electronics unit can be integrated in the rotor end of the flexible printed circuit, coupled to multifunction switches which are arranged on the steering wheel, or to electrical components on the steering wheel side, such as a triggering apparatus for an airbag.
If steering wheel heating is provided, then the control electronics for the steering wheel heating can preferably be integrated in the flexible printed circuit.
The flexible printed circuit also allows coded signals to be transmitted, in which case the signals can be coded and/or decoded, by way of example, by means of integrated circuits on the printed circuit itself.
With regard to the geometric configuration of the device according to the invention, it is advantageous for the stator and the rotor to form an inner and an outer housing element, of which housing elements one surrounds the other in an annular shape, and which can rotate with respect to one another, preferably with the stator surrounding the rotor. The winding element which is provided for holding the flexible printed circuit is either in the form of a component of the rotor, or in the form of a winding segment which can rotate freely. A holder for the flexible printed circuit is thereby formed between the stator and the rotor.
In another embodiment of the present invention, the flexible printed circuit has at least one section on which the direction in which the flexible printed circuit extends is reversed with respect to the circumference of the arrangement. This means that, starting from one end (for example the stator end) of the flexible printed circuit, the flexible printed circuit first of all runs along a first circumferential direction (that is to say in the clockwise sense or in the counterclockwise sense, depending on the nature of the winding) until it reaches said section, at which point the direction in which it extends is reversed (with respect to the clockwise sense).
The reversing section of the flexible printed circuit is essentially U-shaped, and the flexible printed circuit is guided between the stator and the rotor by means of a guide ring, which supports the U-shaped section of the flexible printed circuit. The guide ring forms, between the stator and the rotor, a holder for that part of the flexible printed circuit which is wound up on the winding element, and a further holder for that part of the flexible printed circuit which has been unwound from the winding element.
If the winding element of the device is in the form of a moving winding segment, it is also possible for the winding segment to have a number of U-shaped passages, so that a number of flexible printed circuits can be wound up and unwound one behind the other.
Such an arrangement of a stator, a rotor and the associated electrical conductors is described in principle in EP 0 556 779 A1, DE 195 06 865 C1 and DE 197 34 527 A1, which are referred to here. These embodiments of the invention have the advantage that a comparatively short length of flexible printed circuit is required for the rotation of the steering wheel, and hence of the rotor.
A further embodiment of the invention provides a stator that is intended to hold a control switch, for example for an indicator light switch, with which an electronic assembly on the flexible printed circuit is associated. The control switch may be inserted, in particular, into a holder provided for this purpose in the stator. The printed circuit then forms not only an electronic control unit for components which are supplied with current via the flexible printed circuit, but also for those electrical components, such as an indicator light switch, which are provided on the stator and are supplied with current via separate supply lines for the vehicle electronics.
Further advantages of the invention will become clear from the following description of the exemplary embodiments, with reference to the drawings, in which: